Use the system nanobind instead of a network FetchContent.

Package-manager builds run sandboxed with no network access, so the
FetchContent_Declare(nanobind GIT_REPOSITORY ...) in the rocisa CMakeLists
fails outright.

Regenerated from hipBLASLt-7.1.0-rocisa-nanobind.patch for ROCm 10.0. Note
that 10.0 SPLIT this into two branches: a ROCISA_STANDALONE path that already
calls find_package(nanobind REQUIRED CONFIG) (because scikit-build-core
pre-installs nanobind and injects its CMake path), and an else() path -- the
one taken when rocisa is built as a hipblaslt subdirectory, i.e. ours -- which
still fetches over the network. Only the else() branch needs fixing; this
patch makes it mirror the standalone branch, including finding Python first
as nanobind's config requires.

sci-libs/hipsparselt carries its own copy of the 7.1.0 patch and applies it to
a co-unpacked hipblaslt tree; it needs the same treatment.
--- a/tensilelite/rocisa/CMakeLists.txt
+++ b/tensilelite/rocisa/CMakeLists.txt
@@ -49,16 +49,15 @@
         endif()
         find_package(nanobind REQUIRED CONFIG)
     else()
-        include(FetchContent)
-        FetchContent_Declare(
-          nanobind
-          GIT_REPOSITORY https://github.com/wjakob/nanobind.git
-          GIT_TAG        9b3afa9dbdc23641daf26fadef7743e7127ff92f # v2.6.1
-        )
-        FetchContent_MakeAvailable(nanobind)
+        # Use the system nanobind instead of fetching it over the network:
+        # package-manager builds run sandboxed with no network access. Mirrors
+        # the ROCISA_STANDALONE branch above, which already does exactly this.
         if(ROCISA_USE_STABLE_ABI)
             find_package(Python 3.12 COMPONENTS Interpreter Development.Module Development.SABIModule REQUIRED)
+        else()
+            find_package(Python 3.10 COMPONENTS Interpreter Development.Module REQUIRED)
         endif()
+        find_package(nanobind REQUIRED CONFIG)
     endif()
 
     # HIP is required to build _rocisa (via hip::host).  Set the search prefix